The present invention relates to data storage devices and, in particular, to writing servo patterns on magnetic media.
In a magnetic disc drive, data is stored on one or more discs, which are coated with a magnetic medium. The magnetic medium is typically divided into a plurality of generally parallel data tracks, which are arranged concentrically with one another perpendicular to the disc radius.
The data is stored and retrieved by a transducer or “head” that is positioned over a desired track by an actuator arm. The actuator arm moves the head in a radial direction across the data tracks under control of a closed-loop servo system based on position information or “servo data”, which is stored within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or can be located on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback signal that identifies the location of the head relative to the center line of the desired track. Based on this location, the servo system moves the actuator arm to adjust the head's position so that it moves toward a desired position.
The servo field patterns are typically written onto the disc surface through the product read/write head after the disc has been assembled within the disc drive housing to form a head disc assembly (HDA). A machine called a Servo Track Writer (STW) clamps the HDA along the X, Y and Z axes and then through some method, measures the position of the read/write head and positions the head to the appropriate radial locations to write the servo tracks. Typical methods of obtaining position feedback include the use of a laser interferometer or an optical encoder.
The Servo Track Writer also provides a clock signal with which to align adjacent servo tracks in time. Typically, a clock head is inserted into the HDA and is flown on one of the disc surfaces. The clock head is used to write a clock signal on to the disc surface. This clock signal is then used to run a phase locked loop (PLL) for obtaining a stable reference signal with which to write adjacent radial tracks in a servo pattern with sufficient radial coherence.
There are many advantages to writing the servo tracks in the HDA, after the HDA has been assembled. Unfortunately, the accuracy of the servo tracks (i.e., the actual placement of the tracks on the disc surface) is not optimal when writing the servo tracks in the HDA. Any non-repeatable run-out (NRRO) the HDA at the time of servo writing will be written into the servo pattern. In self-servo track writer systems, the servo tracks are written by bootstrapping the position of each track by the position of the previous track in the HDA with no external references. In these systems, errors in the placement of the servo tracks tend to propagate radially from one track to the next as each track is written in the HDA. Another difficulty encountered when writing the servo tracks is that the accuracy of the servo patterns must increase with decreased track-to-track spacing as the storage density of disc drives continues to increase.
The present invention addresses these and other problems, and offers other advantages over the prior art.